The present invention relates to a sliding nozzle which is secured to a bottom wall of a molten steel receiving vessel such as a ladle or a tundish.
In the prior art, continuous casting of molten steel is carried out in general as follows: Molten steel received in a tundish from a ladle is poured, through a sliding nozzle secured to a bottom wall of a tundish and an immersion nozzle secured vertically to a lower end of the sliding nozzle, into a mold arranged below the immersion nozzle to form a cast steel strand which is continuously withdrawn from the mold as a long cast strand.
The above-mentioned conventional sliding nozzle for a molten steel receiving vessel is disclosed in Japanese Patent Publication No. 1-59,071 published on Dec. 14, 1989 (hereinafter referred to as the "prior art"). The sliding nozzle A of the prior art for a molten steel receiving vessel is explained below with reference to a drawing.
FIG. 1 is a schematic vertical sectional view illustrating the sliding nozzle A of the prior art for a molten steel receiving vessel, which is secured to a bottom wall of a tundish.
As shown in FIG. 1, the sliding nozzle A of the prior art comprises an upper nozzle 1 made of a refractory, an upper fixed plate 2 made of a refractory, a sliding plate 3 made of a refractory, a lower fixed plate 4 made of a refractory, a driving means 5 for the sliding plate 3, and a lower nozzle 6 made of a refractory.
The upper nozzle 1 having a bore 1A is inserted vertically from below into an opening 9A in a nozzle receiving brick 9 provided on a bottom wall 8 of a tundish 7 as a molten steel receiving vessel.
The upper fixed plate 2 having a through-hole 2A and a horizontal lower surface is secured horizontally and stationarily to a lower end of the upper nozzle 1. The through-hole 2A of the upper fixed plate 2 is aligned with the bore 1A of the upper nozzle 1 relative to a common vertical axis.
The sliding plate 3 having a through-hole 3A and having a horizontal upper surface and a horizontal lower surface, is horizontally and reciprocally movable along the horizontal lower surface of the upper fixed plate 2 and a horizontal upper surface of the lower fixed plate 4 by means of the driving means 5 comprising a hydraulic cylinder or the like. The opening of the through-hole 2A of the upper fixed plate 2 is adjusted by causing the sliding plate 3 to slide horizontally, thereby controlling the flow rate of molten steel flowing out from the tundish 7.
The lower fixed plate 4 having a through-hole 4A and a horizontal upper surface is pressed by means of a second fitting means 16 described later through the upper fixed plate 2 and the sliding plate 3 against the lower end of the upper nozzle 1. The through-hole 4A of the lower fixed plate 4 is aligned with the bore 1A of the upper nozzle 1 relative to a common vertical axis.
The lower nozzle 6 having a bore 6A is secured vertically and stationarily to the lower surface of the lower fixed plate 3 by means of a first fitting means 11 described later. The bore 6A of the lower nozzle 6 is aligned with the bore 1A of the upper nozzle 1 relative to a common vertical axis. The lower nozzle 6 has a function of rectifying the flow of molten steel, the flow rate of which has been adjusted by the sliding of the sliding plate 3.
An immersion nozzle 10 having a through-hole 10A, to be connected to the above-mentioned sliding nozzle A, is vertically secured to the lower end of the lower nozzle 6 by means of the first fitting means 11. The through-hole 10A of the immersion nozzle 10 is aligned with the bore 1A of the upper nozzle 1 relative to a common vertical axis. A lower portion of the immersion nozzle 10 is immersed into molten steel poured in a mold (not shown).
The first fitting means 11 comprises a steel shell 12 covering the lower fixed plate plate 4 and the lower nozzle 6, a plurality of long bolts 13 fixed vertically to the steel shell 12 of the lower fixed plate 4, and a ring-shaped holder 14 fitted to a flange 10B of the immersion nozzle 10. According to the first fitting means 11, the immersion nozzle 10 is secured through the lower nozzle 6 vertically to the lower surface of the lower fixed plate 4, by fitting a nut 15 to each of the long bolts 13 running through the holder 14, and tightening the nut 15.
The second fitting means 16 comprises a metal fitting 17 fixed to the bottom wall 8 of the tundish 7, a plurality of arms 19, each being loosely attached to a lower surface of the metalfitting 17 by means of a bolt 18, and a plurality of pins 21, each of which is provided in the metal fitting 17 and is vertically pushed down by means of a spring 20. According to the above-mentioned second fitting means 16, the lower fixed plate 4 is pressed, together with the lower nozzle 6 and the immersion nozzle 10, against the lower end of the upper nozzle 1 through the sliding plate 3 and the upper fixed plate 2, by pushing down one end of each arm 19 by means of the pin 21 under the action of an elastic force of the spring 20 so as to push up the other end of the arm 19 with the bolt 18 as a fulcrum, thus pressing up the lower surface of the lower fixed plate 4.
According to the above-mentioned sliding nozzle A of the prior art, it is possible to control the flow rate of molten steel which is poured from the tundish 7 through the upper nozzle 1, the sliding nozzle A, the lower nozzle 6 and the immersion nozzle 10 into the mold (not shown), by causing the sliding plate 3, by the actuation of the driving means 5, to slide horizontally along the horizontal lower surface of the upper fixed plate 2 and the horizontal upper surface of the lower fixed plate 3, thereby adjusting the respective openings of the through-hole 2A of the upper fixed plate 2 and the through-hole 4A of the lower fixed plate 4.
However, the above-mentioned sliding nozzle A of the prior art has the following problems:
(1) In order to secure the sliding nozzle A to the tundish 7, it is necessary first to secure the immersion nozzle 10 through the lower nozzle 6 vertically to the lower surface of the lower fixed plate 4 by means of the first fitting means 11, and then to press the lower fixed plate 4, together with the lower nozzle 6 and the immersion nozzle 10, against the lower end of the upper nozzle 1 through the sliding plate 3 and the upper fixed plate 2 by means of the second fitting means 16. Securing of the sliding nozzle A to the tundish 7 is therefore intricate and time-consuming.
(2) Mortar, which is applied onto the junctions between the lower fixed plate 4 and the lower nozzle 6 and between the lower nozzle 6 and the immersion nozzle 10, contracts under the effect of heat received from molten steel, and becomes brittle. As a result, gaps are produced in the above-mentioned junctions, thus leading to a lower tightness against molten steel at the abovementioned junctions.
(3) Thermal expansion of the long bolts 13 and the ring-shaped holder 14 of the first fitting means 11 results in a lower securing function of the first fitting means 11, thus leading to a lower tightness against molten steel at the above-mentioned junctions.
(4) The arms 19 of the second fitting means 16, which locally press the lower surface of the lower fixed plate 4, tend to cause breakage of the lower fixed plate 4.
Under such circumstances, there is a strong demand for the development of a sliding nozzle for a molten steel receiving vessel, which permits an easy securing to a tundish, has an excellent tightness against molten steel, and is free from breakage of a lower fixed plate, which breakage tends to occur under the effect of a fitting means locally pressing the lower fixed plate, but such a sliding nozzle has not as yet been proposed.